Static random access memories (SRAMs) are used in numerous applications in which access time is important. A typical memory is constructed from a plurality of SRAM cells connected to bit lines used to read from, and write data into, particular cells. A typical SRAM cell consists of 4 transistors connected as a flip-flop and two transistors that are used to connect the flip-flop to the bit lines.
The bit lines are often quite long, and hence, have a significant capacitance. To write data into an SRAM cell, the potential of at least one of the bit lines must be changed by an amount that is typically equal to the supply voltage. For example, if a "0"is to be written into a memory cell in a system in which the bit lines are precharged to some reference potential, the bit line must swing from the reference potential to zero to write the data and then back to the reference potential. The time needed to complete this swing is large because of the large capacitance of the bit line. Hence, if a read operation, i.e., an operation requiring the bit lines to be precharged, is performed after a write, the read requires more time to complete, because the system must wait for one of the bit lines to return to the precharge potential.
In addition to increasing the time needed to read data after a write, the need to swing the bit lines through the full voltage range also requires a significant amount to power. The power consumption is proportional to the voltage swing on the bit lines. In memories having large word lengths, the number of bit lines being written simultaneously may be large. Hence, power consumption during write operations can be a significant problem in such memories.
Broadly, it is the object of the present invention to provide an improved SRAM:
It is a further object of the present invention to provide an SRAM in which the bit lines are not required to swing through the full voltage range during a write operation.
It is a still further object of the present invention to provide a SRAM with reduced power consumption.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.